WLAN technologies constitute a fast-growing market in which wired connections between communicating devices in a local network, e.g. in an office, home or production environment, are replaced by wireless connections. Advantages of WLANs compared with wired networks include greater flexibility to communicate without restriction, minimal need for previous planning work to construct the network and greater robustness of the network.
Many WLANs operate in accordance with an industry standard which defines a protocol for communication between nodes or terminals of the WLAN. The 802.11 standard of the IEEE (IEEE, 1997) is an example of such a standard. Options permitted by the 802.11 standard for the ‘physical layer’, i.e. the wireless communication medium, are infrared and spread spectrum radio transmission techniques. The 2.4 GHz ISM (Industrial, Scientific and Musical) band, which is available in most countries around the world, and the 5 GHz band, may be used for the radio communication options.
WLANs such as those operating in accordance with the 802.11 standard can exhibit one of two different basic network architectures, namely ad hoc and infrastructure-based network architectures. Ad hoc architectures include several client nodes using the same frequency for communication without an infrastructure. Infrastructure-based architectures include an infrastructure node, called in the 802.11 standard an ‘Access Point’ or ‘AP’, which may for example comprise a connectivity point to a central computer processor and/or to network distributed services. Each client node, called in the 802.11 standard a ‘Station’, or ‘STA’, is operably connected to the network via an AP. STAs are nodes or terminals with access mechanisms to the wireless medium giving connectivity to the AP. The STAs may for example be connectivity points to computer peripheral devices. The STAs and the AP which are within the same wireless coverage area form a basic service set (‘BSS’). Multiple BSSs may be connected together via their APs in a distribution system forming an enlarged network.
Typically, the design of infrastructure-based WLANs is simpler than alternative WLAN networks which are ad hoc networks. This is because in the infrastructure-based network most of the network functionality lies within the infrastructure node, i.e. within the AP in a WLAN operating in accordance with the 802.11 standard. In comparison, the client nodes, e.g. the STAs in a WLAN operating in accordance with the 802.11 standard, can remain relatively simple.
Many devices employed as or in association with client nodes in a WLAN are battery powered. Therefore, power-saving mechanisms are used in order to reduce power consumption in such devices. WLAN standards such as 802.11 assume that client nodes are always ready to receive data, although their receivers are idle for most of the time in lightly loaded networks. However, this permanent readiness to receive data causes considerable drain of battery energy. In WLANs operating in accordance with the 802.11 standard, the STAs are allowed to be switched off when they are not in active use, in order to preserve battery energy.
Thus, the basic principle of power saving in a WLAN operating in accordance with the 802.11 standard includes each STA having two states or modes, namely (i) a ‘sleeping’, or ‘PS’ (power saving) state or mode and (ii) an ‘awake’ state or mode. Each STA has its receiver (and other components such as its transmitter) active (i.e. switched on) only in the awake state but not in the sleeping state. If a sending terminal intends to communicate only with a particular individual target STA by a so-called ‘unicast’ communication, the communication is routed via the AP which serves the target STA (this AP may not however be the one serving the sending STA). The target STA gives prior notification to its serving AP of its intention to enter the sleeping state. This is done during an association procedure between the STA and the AP. In this procedure, the STA also negotiates with the AP a period for which the serving AP will store (buffer) for the STA, whilst the STA is in the sleeping state, data messages, known in the art as data ‘frames’, comprising unicast data communications destined for the STA as a target STA. The STA then enters its sleeping state, and the serving AP stores (buffers) any data message(s) comprising the unicast communication(s) to be provided to the target STA for the period which has been negotiated. That STA (as well as any other STA in a sleeping state) periodically has to be switched into its awake state and to stay in its awake state until a ‘beacon’ signal is sent from the serving AP for the STA and then act accordingly.
Each AP sends the beacon signal by broadcast transmission to its associated STAs. The beacon signal includes amongst other things an announcement message including an ‘Information Element’ (IE), known as a ‘Traffic Indication Map’, or ‘TIM’, including a list of STAs for which unicast data messages have been temporarily stored in the AP. The TIM within the beacon signal is sent periodically. If a STA detects from the TIM that it is a target destination of a stored data message held by its serving AP, it has to request the AP to send the stored data message(s), then the STA must stay awake until receipt of the data message is completed.
Waking up, i.e. being switched into the awake state, at the right moment by each STA requires use of a timing synchronization function (TSF) as defined in the 802.11 standard. This function ensures that all STAs are awake together using the same timing sequence, by providing preliminary knowledge regarding the expected point in the timing sequence at which the AP is about to transmit the announcement message including the TIM. Implementation of procedures to provide this preliminary knowledge is done in different ways by different product manufacturers.
Additionally, each AP provides a Delivery Traffic Indication Message (DTIM). The DTIM is another ‘Information Element’ (IE) of the announcement message sent periodically within the beacon signal. The DTIM indicates that there is at least one multicast transmission, i.e. a transmission to be sent to a plurality of STAs (not to a single target STA as in a unicast transmission) and/or at least one broadcast transmission to be sent to all STAs associated with the given AP. Data messages comprising the multicast and/or broadcast transmissions follow directly after each announcement message containing the DTIM. These data messages are also known as ‘MAC (Medium Access Control) Service Data Units’, or ‘MSDUs’. STAs in known systems are always required to be in their awake state for announcements about multicast and broadcast data frames and for delivery of such data frames following the announcements.
The period between successive DTIMs, known as the ‘DTIM interval’, is always a multiple of the period between TIM messages, known as the ‘TIM interval’. All STAs in known systems wake up, i.e. are switched to their awake state, prior to an expected DTIM. STAs are allowed to return to their sleeping state after receipt of a DTIM and an MSDU transmission following it, following a TIM in which no stored unicast data frame has been indicated for the STA, or following announcement in a TIM of an indication of at least one stored unicast data message and completion of receipt of the indicated at least one message.
Procedures which have been proposed in the prior art for switching STAs between their sleeping and awake states may provide valuable power saving in each STA. However, the present inventors have recognised in relation to the present invention that it would be desirable to provide further power savings not hitherto contemplated.